CT systems and methods are widely used, particularly for medical imaging and diagnosis. CT systems generally create images of one or more sectional slices through a subject's body. A radiation source, such as an X-ray tube, irradiates the body from one side thereof. A collimator, generally adjacent to the X-ray source, limits the angular extent of the X-ray beam, so that radiation impinging on the body is substantially confined to a planar region defining a cross-sectional slice of the body. At least one detector (and generally many more than one detector) on the opposite side of the body receives radiation transmitted through the body substantially in the plane of the slice. The attenuation of the radiation that has passed through the body is measured by processing electrical signals received from the detector.
The development of photon counting (PC) detectors in CT applications has enabled a new dimension of CT imaging, namely “spectral CT” or “multi-energy CT.” In a spectral CT system, typically multiple X-ray sources are provided, each having a respective detector positioned opposite thereto such that X-rays may be emitted from each source having different spectral energy content. Once multi-energy data is obtained, a pre-reconstruction decomposition algorithm may be applied in order to image two distinct materials, such as water and iodine. The pre-reconstruction decomposition algorithm may be based on the concept that, in an energy region for medical CT, the X-ray attenuation of any given material can be represented by a proper density mix of two materials with distinct X-ray attenuation properties, referred to as the basis materials. The pre-reconstruction algorithm computes two material density images that represent the equivalent density of one of the basis materials based on the measured projections at high and low X-ray photon energy spectra, respectively. The material density images may be further converted to form monochromatic images, density images, or effective-Z images.
However, noise in the monochromatic images, density images, and effective-Z images is propagated during the decomposition process, and the noise is typically correlated. In other words, noise generated in both low and high kVp acquisitions typically correlates during pre-reconstruction decomposition and propagates in subsequent generation of basis material images.